For the power control of nuclear reactors and for the emergency stoppage of these reactors in the event of a breakdown, assemblies of elements absorbing the neutrons are used which are displaced between the fuel elements in the core of the reactor, in order to increase or to reduce their insertion inside the core, which tends to reduce or to increase the power emitted by the core.
The displacement of these assemblies of absorbing material of great length is generally effected by means of a drive element for the absorbing assembly, consisting of a rod of great length connected at one of its ends to the absorbing assembly and disposed coaxially in relation to this assembly.
In the case of pressurized water reactors, in particular, each absorbing assembly consists of tubes enclosing a material which absorbs the neutrons and is inserted vertically inside the fuel assemblies, constituting the core of the reactor, to a greater or lesser depth, as a result of a control rod disposed vertically, and to the lower portion of which there is fixed the absorbing assembly called the control rod, this control rod being displaced vertically in a precisely controlled manner by means of a mechanical or electromagnetic device.
In order to obtain the emergency stoppage of the reactor, the control rod is released so that this rod and the absorbing element which is connected thereto fall into the core of the reactor in their position of maximum insertion under the effect of gravity.
The control rod is generally connected, by its lower portion, to the upper portion of the absorbing assembly by means of a device consisting of at least two deformable resilient metallic parts disposed at the base of the control rod and a spacing member fixed to the end of a central rod, disposed along the axis of the control rod and the purpose of which is to maintain in position the resilient metallic parts in seatings machined in the upper portion of the absorbing element.
When the spacing member is held at the level of the resilient parts, these parts cannot approach one another under the effect of the weight of the absorbing element when this element, suspended from the control rod, is displaced vertically.
In prior art devices, the spacing member has to be displaced in axial translation in order to be able to release the absorbing element and to separate it from its control rod. In fact, if the spacing member is moved away, by axial translation, from the deformable attachment members for the absorbing element, the weight of the absorbing element, on an upward movement of the control rod, causes the deformation of the resilient attachment parts, and these deformable parts are designed so as then to slide inside the seatings of the absorbing element and release this element, the movement of the control rod continuing without the absorbing assembly being entrained.
Nevertheless, in order to effect the displacement of the spacing member by axial translation of the central rod, it is necessary to have direct access to the control rod to effect the movement of vertical translation manually.
In the prior art devices, particularly for pressurized water reactors, this operation can only be carried out after the reactor has been stopped and depressurized, and after either the cover of the tank under pressure containing the nuclear core, or the sealed casing of the mechanism controlling the controlled displacement of the control rod, has been dismounted.
It is necessary to proceed thus, particularly during the operations of recharging the reactor with fuel, because it is necessary to separate the control rods and the absorbing elements prior to these operations. In pressurized water reactors, this separation operation takes a fairly long time, which prolongs the period of stoppage of the reactor, and requires manual intervention in an atmosphere which may be contaminated, which makes it necessary to take particularly onerous and constricting precautions.
On the other hand, in the course of these recharging operations, both in the case of pressurized water reactors and in the case of high-temperature reactors or fast-neutron reactors cooled by liquid metals, it may be of interest to dispose the control rods in a high position after having uncoupled them from the absorbing assemblies. The operations of replacement of the fuel are thus greatly facilitated either at the moment of lifting the tank cover, in the case of pressurized water reactors, or during the actual recharging in the case of high-temperature reactors or fast-neutron reactors where the roof the tank remains in position during the recharging.
To date, no coupling or uncoupling device was known for two elements of great length, such as a control rod and an element absorbing the neutrons of a nuclear reactor, which permits operation in a reliable, automatic and remote manner, i.e. in all the cases where it is not easily possible to have access to the junction zone between the elements, or even when this access is impossible and it is necessary to operate in a perfectly sealed manner.